Mask controlled signal generating system



Jan. 19, 1960 D. E. sUNsTElN MASK CONTROLLED SIGNAL GENERATING SYSTEMFiled May 6, 1955 3 Sheets-Sheet 1 l faz Q ll l I /f j i l g |52 l @gf I-f e 0f f ,576.57 Y s2 at?y I f4 .ff Z. Il l I 6. 6. I

INVENTOR.

D, E. SUNSTEIN MASK CONTROLLED SIGNAL GENERATING SYSTEM Jan. 19, 1960Filed May 6, 1955 si nl? 7@ Mr 2 EN@ a M m wu. ma W 7. ffm l. Aol/.E iwww Vff Mp/ i.. www 3 IHM A z Y i M R m 7 4 Jan. 19, 1960 D, E, sUNsTElN2,922,049

MASK CONTROLLED SIGNAL GENERATING SYSTEM Filed May 6, 1955 ssheets-sheet s rates MASK CONTROLLED SIGNAL GENERATING SYSTEM David E.Sunstein, Bala-Cynwyd, Pa., assignor `to Philco Corporation,Philadelphia, Pa., a corporation vof Penn- Sylvania Application May 6,1955, Serial No. 506,461

Claims. (Cl. Z50-217) This invention relates generally to improvementsin function generators and more particularly to function generatorsemploying structures which operate to produce a signal whose magnitudevaries in accordance with variations in the path of a mask such as aline mask or a moving spot mask.

There are many applications for yfunction generators in the art. Many ofthese uses are set forth in U.S. Patent 2,528,020 issued to David E.Sunstein October 3l, 1950 which describes a specific function forobtaining an electrical signal which can be related to another signal byany desired mathematical curve or expression. More particularly,function generators can be employed to generate electric waveforms ofarbitrary shape such as a waveform representative of a particular sound.Also, arithmetic operations such as multiplication, division, andsquaring may be performed through the use of function generators. Otherfunctions include the solution of algebraic equations, Fourier analysisof waveforms, scrambling and amplitude modulation. Reference is made tothe Sunstein Patent 2,528,020 for a more detailed account of these andother uses for function generators.

Excluding the structure of the above-mentioned Sunstein patent, whichdiscloses means whereby the electron beam of a cathode ray tube iscaused to follow the edge of a mask positioned in front of the tubescreen, there are many dilerent structures in the prior art which may beemployed as function generators. Some of these are, for example, diodes,crystals, electronic tubes and electronic circuits. All of thesestructure exhibit the common characteristic of non-linearity betweenvoltage and current. In these structures this non-linearity is due tonatural characteristics peculiar to the particular structure inquestion. For the most part these natural characteristics follow noparticular mathematical law. Some of them approximate particularmathematical laws such as square law, for example, but not so closelythat considerable improvement is not possible. Similarly, others mayhave natural characteristics which approximate other relationships toperform other functions such as linear detection. In some instancescertain variations and modiications of these non-linear circuit elementscan be made which will improve the approximation of the desiredmathematical relation. Howerever, all of these prior art devices,excluding the structure disclosed in Sunstein Patent 2,528,020, arelimited in one or more ways. A common limitation has been one of minimumand maximum voltage below and above which the amount of error in theapproximations becomes quite large. For example, in linear detectors,the detector functions quite well as long as the voltage to be detectedis above a certain minimum value. lf the voltage to be detected dropsbelow such a minimum value, the detection will usually cease to belinear, and will begin to exhibit distortion. A further limitation ofprior art non-linear circuit elements is the difficulty in controllingthe relationship between a pair of voltages within a circuit.

There are, however, devices in the prior art which.

overcome these limitations and provide for the generation of functionalsignals which will follow a given mathematical law to a very closeapproximation. These devices are of the type shown and described in theaforementioned Sunstein patent and are sometimes referred to asphotoformers. The particular structure described therein constitutes anon-linear circuit element whose characteristic curve can be controlledat will. More speciiically there is disclosed a function generator inwhich a ymask is positioned between the luminous screen of the tube anda photoelectric element. The shape of this mask determines thecharactertistics of the device. The electron beam of the cathode raytube is locked upon the edge of the mask by means of afeedback circuitextending from the photoelectric element to the deflection plates of thecathode ray tube. In operation the spot of light produced on the screenof the cathode ray tube by the electron beam impinging thereon will seeka stable position wherein a portion thereof which otherwise would fallon the light responsive device will be intercepted by said mask. This isgenerally accomplished by two different and opposing signals which, whenapplied to a pair of the deflection plates of the cathode ray tube (as,for example, the Vertical deliection plates) tend to cause deflection ofthe electron beam in opposite directions. More specically there is afirst signal applied to the vertical deflection plates which is a normalbiasing signal and which tends to deect the electron beam in a firstdirection which will be assumed to be upwards. The second signal isderived from the light responsive device and supplied to the verticaldeection plates of the cathode ray tube through said feedback means.This second signal creates an electric field which tends to deflect theelectron beam in a downward direction. The parameters of the circuit areselected so that Ysaid second signal causes a stronger force to beexperted on said electron beam than does said first signal when thelight responsive device is exposed to the entire spot of light.Consequently, stability Will be obtained between those opposing forceswhen a sufficient amount of the spot of lightl is cut off by the mask.If the electron beam should attempt to rise above the mask (assuming thenormally exposed portion of the light to be above the mask) theincreased light impinging upon the light responsive device will produce,through the feedback means, a signal which will tend to return theelectron beam downward towards the mask. If, on the other hand, theelectron beam should move downward so that too much of the light is cutof from the light responsive device by the mask, then the normal bias inthe system will cause the electron beam to move upward towards the edgeof the mask until the proper amount of light is permitted to impingeupon the light responsive means.

In one form of the prior art the mask is in the form of a sheet havingone of its edges shaped in accordance with a desired function. Thepreparation of such a mask is relatively difficult compared to thepreparation of, for example, a line mask which is employed in some priorart and which may be formed simply by drawing an opaque line on a pieceof transparent material such as glass or plastic. The use of a linemask, however, in prior art devices involves certain disadvantages inthat the electron beam may drop below the line mask due to some fault.Thesignal fed back to the vertical deliection plates from the lightresponsive device will tend to deliect the electron beam to the bottomof the screen. Thus, should such a fault occur, additional circuitrysuch as a trigger circuit, for example, isy required to deliect theelectron beam back up above the line mask. After cessation .of the eiectof the trigger circuit, the electron beam will be deflected down to theline mask by the signal fed back r 2,922,049 o s y from the lightresponsive device, and should Ybecome Y faulty operationdueeto theelectron beam lleaving the line Y mask, where such a mask is employed,is not present.

In another form of the prior art shown and described in United StatesPatent 2,455,532; issued December 7,

1948, to David E. Sunstein, the electron beam is caused to follow amoving point. This is accomplished by positioning a mask divided intofour quadrants between the screen of the cathode ray tube and the lightresponsive means which include two photosensitive devices. Individualfilters are positioned between each of the two photosensitive devicesand the mask. The fourquadrants of the mask and the filters arepolarized, made transparent, or made opaque so that, to one of the twophotosensitive devices,.the quadrants below the abscissa of the quadrantmask will appear opaque and thetquadrants above the abscissa will appeartransparent, while, to the other of the two photosensitive devices,'thetwo quadrants on one side of the ordinate will appear opaque while thetwo quadrants on the other side of thefordinate will appear transparent.Means similar to that described hereinbefore are provided to feed backthe signalsV from the two photosensitive devices to the horizontal andvertical deection elements to maintain the electron beam at theintersection of the abscissa and the ordinate.V

The masking and lter arrangement described above, however, arerelatively elaborate. Furthermore, the use of two photosensitive devicesis expensive and space consuming. It would be an improvement in the artif lthe same result could be obtained with only one photosensitivedevice and without the arrangement of masking and iilters.

An object of the invention is to provide a function generator which willproduce a signal 'Whose magnitude varies in accordance with thevariations in the path of a masking element.

Another object of the invention is to provide a reliable functiongenerator which will produce a signal whose amplitude varies inaccordance with the variations of a line masking element. Y

t A further object is to provide a reliable function generator ofrelatively simple structure which will produce signals indicative of theposition of a moving spot mask.

A fourth object of the invention is the improvement of functionvgenerators'generally.

In accordance with the invention there is provided means for generatinga signal which bears a known relationship to the path of a maskingelement. This means may comprise a cathode ray tube, a luminousV screen,

means Yfortgenerating an electron beam, and deilectingY means responsiveto dellecting signals applied thereto to cause said electron beam tocyclically sweep across said screen in at least one coordinate and abouta nominal mid-point. A light responsive device is exposed to the lightfrom said luminous screen and constructed to produce an output signalwhose magnitude varies in accordance with variations in the intensity ofthe light from said screen. In the invention such variations are causedby the masking element which vis positioned between the screen of thetube and the light responsive device. Means are provided to combine theoutput signals from said light responsive device and said deflectingsignals to produce an output signal whose amplitude variesfin accordancewith the distance in said one coordinate between the midpoint of saidsweep of the electron beam and the position of that portion of themasking element path -intercepting sweep of the electron beam to be'deflected ltoward theV position of the portion of the mask beingscanned until the distance therebetween is of the proper value tomaintain the new position of said mid-point.

In accordancewithl another feature of the invention, in which signalsare generated in accordance with the position of a spot mask, separatemeans are provided to supply rst and second deecting signals to theelectron beam deflecting means to cause the electron beam to sweepcyclically across the screen in a iirst coordinate and secondcoordinate. Individual means are provided to separately combine thesetwo'deflecting signals with the output signal'from said light responsivemeans to produce output signals whose amplitudes vary in accordance withthe position of the spot mask in the first and second coordinates.

These and other objects and features of the invention will be more fullyunderstood from the following detailed description thereof when read inconjunction with the drawings in which: l

Figure 1 is a combination perspective view and block diagram of a formof the invention;

Figure 2 is a schematic diagram of one of the block elements of Figure1; A

Figures 3, 4,' 5 and 6 show voltage waveforms at various points of thecircuit yof Figure 1;

Figure 7 shows a combination perspective view and blockdiagram ofanother form of theinvention;

Figures 8, 9, 10 and 11 illustrate voltage waveforms at various pointsin the circuit of Figure 7;

Figure 12 shows a combination perspective view and block diagram ofanother embodiment of the invention; and

Figure 13 shows a typical scanning pattern `which may be utilized in thecathode ray tube of Figure'12.

Referring now to Figure l, the signal source 21 may be constructed togenerate a Vsignal having anyA convenient w-aveform such as a sawtoothwaveform or a sine A wave in accordance with the functionedesired. Thissigthe light between said screen and saidlight responsive l naly issupplied to horizontal deilecting plates 22 of the cathode ray tube 23which also comprises vertical deecting plates 24, luminous screen 25,cathode 26, focusing electrode 2,7, and accelerating electrode 34.Batteries 35 and 36 are employed to bias electrodesl 34 and 27respectively with respect to cathode V26. Signal source 28, which alsomay be constructed to generate a signal having any convenient waveformsuch as a sawtooth waveform or a sine wave, is arranged-ton supply` sucha signal to the vertical deflection plates 24y of the cathoderray tube23, through adding circuit 29. The electron beam of the cathode ray-tubel23 thus will have a raster determined bythe signals supplied to thehorizontal deection plates 22 andthe vertical deection plates 24 bythe'signal sources 21- andv28 respectively. A light responsive device30, which may be a photoelectric cell preferably of theelectron-multiplier type, alsoreferred to herein as a photoelectricelement or a photosensitive element or device, is positioned so as to beexposed to the light appearing on the screen 25 of the cathode ray tube23. lThe signal output from the light responsive device 30 is suppliedto a phase comparator circuit 31. Also supplied to the phase 'comparatorcircuit 31 is the signal from the signal source 28. The output of thephase comparator circuit is a signal whose amplitude varies inaccordance with the position of the portion of the line mask 32 beingscanned, as will be discussed indetail later. This signal is Vfed backto rthe vertical deection plates 24 through the adder circuit 29.y ,g 1

As stated hereinbefore, the general function of the inf vention is-toproduce an output signal in accordance with the path of a mask. InFigure 1 the mask is a line mask represented by the referencecharacter32. During those periods of time when the light produced on the screen25 by the electron beam is not cut offrom the photoelectric element ,30by the line mask 3 2, there.v will be a substantially constant signaloutput produced on conductor 99 by the photoelectric element 30.However, each time the beam passes by the mask 32 so that the spot oflight is cut oi from the photoelectric element, a pulse will be producedon conductor 99 by the photoelectric element 30. These pulses will occurat time intervals determined by the position of the mask being scannedand the midpoint of the sweep of the electron beam. v

Assume that, in the absence of a signal fed back to the verticaldeiiection plates from the photosensitive element 30, the mid-point ofthe vertical sweep of the electron beam is caused to coincide with thehorizontal center line 45 of the screen 25. This mid-point is alsoreferred to herein as the zero crossing of the vertical deiiectionsignal. Such terminology applies to all figures of this specification.If the portion, such as portion 37, of the line mask, past which theelectron beam is sweeping at any particular time, also lies in thecenter line of the screen 25, then the electron beam will sweep pastthis portion of the line mask at regularly spaced time intervals at afrequency rate equal to twice the frequency of the signal from source28. Under such circumstances, pulses will be generated by thephotoelectric element 30 at regularly spaced time intervals and having afrequency equal to twice that of the signal from source 28. -If theportion, such as portion 3S, of the line mask 32, past which theelectron beam is sweeping at any particular time, lies above the centerline 45 of the cathode ray tube screen, then the beam will sweep by thatportion of the line mask at irregular time intervals since the beam issweeping past the line mask at points in time above the zero crossing ofthe signal from the source 28. Similarly, if the electron beam issweeping across a portion, such as portion 39, of the line mask 32 whichlies below the center line of the screen, it (the electron beam) willpass the line mask at irregularly spaced intervals.

It is inherent in the system of Figure 1 that, if the output pulsesproduced by the photoelectric cell 30 are ir regularly spaced withrespect to time, there will be a waveform component therein which hasthe same frequency as the signal from source 28 and whose phase iseither equal to the phase of the signal from source 28 or 180 out ofphase with the signal from source 28 depending upon whether the linemask is above or below the center line of the screen.

The phase comparator circuit 31 is constructed to compare the signalyoutput from said photoelectric cell 30 with the signal from said signalsource 28 to producean output signal on conductor 18 whose polarity andamplitude depend upon the relationship of the phase of the pulses fromthe photo-electric cell 30 with respect to the phase of the outputsignal from source 28. Amplifier 33 performs the function of amplifyingthe output of the phase comparator 31 which appears on conductor 18.Such amplification is necessary to obtain a signal of sufficientmagnitude to deiiect the mid-point of the vertical sweep of the electronbeam to a new position in accordance with the position of the portion ofthe mask being scanned, and to maintain it in these positions. Addercircuit 29 performs the function of adding the output from the amplifier33 to the signal from source 28 to produce an output signal which, whenapplied to the vertical deiiection plates 24, will cause the zerocrossings thereof to be shifted towards the portion of the line mask 32being scanned. It is to be noted that, in its new position, themid-point of the vertical sweep will not occur precisely at the linemask inasmuch as some physical difference therebetween (in the plane ofthe screen) is required in order to obtain an output signal from thephase comparator 31. Such an output signal is necesary to maintain thesaid mid-point in its new position.

The amount of this physical difference is determined by the totalamplification in a circuit loop extending from the photoelectric cell 30through the phase comparator circuit 31, amplier 33, adder circuit 29,and cathode ray 6 tube 23 back to the photoelectric cell 30. Forexample, assume that volts is required to deiiect the electron beamvertically one inch in the plane of the screen, and that the totalamplification factor of the circuit loop described hereinbefore is 99.Further, assume that a difference of a hundredth of an inch between theline mask and the zero crossing of the vertical sweep of the electronbeam will produce one volt output from the phase comparator 31. It canbe seen, then, that, if the line mask is one inch above the center line45 of the screen 25, the system will be balanced when the mid-point ofthe vertical sweep is one hundredth of an inch below the line mask,since one hundredth of an inch will produce one volt output from thephase comparator circuit 31. This one volt output will beamplified to 99volts by ampliier 33, which will deflect the mid-point of the verticalsweep of the electron beam ninety nine hundredths of an inch. The outputof the circuit may be taken from the output of the amplifier 33 andsupplied to a load 19.

In Figure 2 there is Shown a schematic sketch of a phase comparatorwhich can be employed in the block element 31 of Figure 1. ConsideringFigure 1 and Figure 2 together, the signal from source 28 of Figure 1 isimpressed across the primary winding 101 of the transformer 102 ofFigure 2. Element 103 offFigurev 2 corresponds to the light responsivedevice 30 of Figure 1. The potential appearing between the lead 44 andgrounded lead 53 corresponds to the output signal appearing on theconductor 18 of Figure 1. Anodes 98 and 58 of the diodes 148 and 149 areconnected together through the secondary winding 57 of transformer 102.The cathodes of diodes 1-48 and 149 are connected together through equalvalued resistors 40 and `41 which shunt equal-valued capacitors 42 and43 respectively. The capacitors 42 and 43, in cooperation with theassociated resistors 40 and 41, perform the function of detecting thepulses of ctu'rent iiowing through the tubes 14S-and 149 respectively,as will be discussed in detail later. Resistors 40 and 41 have highvalues to provide a discharge time constant for the associatedcapacitors which is relatively large with respect to the frequency ofthe signal applied to the primary winding 101 of the transformer v102and the signal from the element 103. The element 103 is connectedbetween the mid-point of the secondary winding l57 of transformer 102and the mid-point between the equal resistors 40 and 41.

The operation of the circuits of Figures 1 and Z will nowv be described.First, there will be described the operation of Figure 2 with thewaveform of Figure 3 supplied to the primary Winding 10'1 of transformer102 (Figure 2) but without a signal being supplied from thephotoelectric cell 103. Then,.the operation of both Figure 1 and Figure2, with both of the above mentioned signals supplied, will be describedunder three different sets of conditions. These three conditions are:first, when the electron beamis sweeping by a portion of the line mask32 which is at the center line y45 of the screen 25; second, when theelectron beam is sweeping by a portion of the mask which is above thecenter line of the screen; and third, when the electron beam is sweepingby a portion of the mask which is below the center line of the mask.

As described hereinbefore, the circuit of Figure 2 is balanced.Therefore if no signal is supplied from the element 103, the tubes 148and 1-49 will alternately conduct equal amounts of current in responseto the alternating signal of Figure 3 which is supplied to the pr-imaryWinding 101 of the transformer 102. This will produce equal but opposingD.C. voltages across the capacitors -42 and 43 Thus there will be nooutput signal across the leads 44 and 53. More specifically, assumelthat the positive portion 54 of the waveform of Figure 3 will produce apositive-voltage at terminal 55 of the transformer 102. Terminal ofsecondary winding IS7 will then be at a negative potential with respectto termi- Vnal 55. Under these circumstances the diode '148 willbeconductive through a circuit which may be traced through the diode148, resistance 40, photoelectric cell 103 'and the upperhalf of winding57. This will produce a potential across thel capacitor -42 with thepositive plate being connected to the conductor 44. Diode 149 will notbe conductive because the potential of the anode 58 thereof is negativewith respect to the potentialrof the cathode. Consequently, thecapacitor 43 will acquire no potential thereacross. When the negativeportion 59 of the waveform of Figure 3 is supplied to the primarywinding 101 of the transformer 4102, the terminal 105 of the secondarywinding thereof will be at a positive potential, whereas the terminalV55 will be at aV negative potential. Under these conditions the diode149 will be conductive Vin a circuit extending through the diode 149,resistance 41, photoelectric cell 103, and the lower portion of thesecondary winding 57. The tube 148 will not be conductive inasmuch asits anode 98 is at a lower potential than its cathode. 42 will acquireno potential thereacross during this half cycle, whereas the capacitor43 will acquire a negative potential on that plate which is connected tothe photoelectric :cell v103. However, the absolute potential acquiredby capacitor V43 during the negative half of the cycleis equal'to theabsolute potential acquired vby the capacitor 42 during the positivehalf of the cycle. Thus, since the time rates of discharge of capacitors42 and 43 areequal,` and further, since said time rates of discharge arerelatively slow compared to the frequency of the signal supplied to theprimary winding 101 from the vertical deection sweep signal source 28(Figure l), theoverall effect of the capacitors 42 and 43 is to produceno D.C. signal between theconductors 44 and 53.

Assume now that the electron beam vis sweeping past a portion 37 of theline mask which is at the center line 45 of the cathode ray tube screen.The potential across the vertical dellection plates 24 is normallyregulated so that the zero crossings of the vertical sweep of, theelectron beam will coincide with the center line 45 of the screen in theabsence of a signal from the amplifier 33. Thus the electron beam willsweep past the portion 37 of the line mask 32 at intervals of timecorresponding to the zero crossings of the signal from the sourc e`28.The signal from source 28 is represented by the waveform of Figure 3,and the zero'crossings of said waveform are identified by the referencecharacters 47, 48 and 49. The resultant pulses produced by thephotoelectric cell 30 (Figure 1) are indicated by reference characters50, 5'1 and 52 of Figure 4 and occur at a -frequency twice that of thewaveform of Figure 3. It canebe shown by waveform analysis that a firstrecurring waveform, having a frequency which is twice the frequency of asecond` recurring waveform, has no component thereof which is equal tothe frequency of said second recurring waveform. Such a component isrequired in order to obtain an output signal from the phase comparatorcircuit 31. This may be seen more clearly from the following discussion.Since thefrequency of the waveform of Figure 4vis twice that of thefrequency of the waveform of Figure 3, any

Consequently the capacitor` two consecutive pulses lof the waveform ofFigure 4, such as pulses 51 and `52, will coincide in time withcorresponding portions of adjacent half cycles, such as positive halfcycle 5,4 and negative half cycle 59, of the waveform of Figure 3.However, the positive half cycle 54 represents a positive increase inthe potential of the plate 98k of tube 148 of Figure 2, and the negativehalf cycle 59 represents a positive Vincrease in the potential of theplate 53 of tube 149, whereas =both of the pulses'51 and 52 of Figure 4cause simultaneous decreases in the potentials of the cathodes of tubesk148 and 149. Consequently the total current conducted by; said tubes148 and '149 will be increasedin accordance therewith. However, sincethe pulses 51 and 52 occur in corresponding parts of the half cycles 54and 59 of Figure 3 the over-all elfect'is that the total currents of thediodes 148 and '149 are increased by equal amounts. Therefore the netpotential accumulated by the capacitors 42 and 43 is similarly increasedby equal amounts, and lthe D.C. potential existing between conductors 44and 53 remains zero. Thus in FigureV l, the QD..C potential appearing oncorresponding output terminal 18 from the phase comparator 31 is zeroand the midpoint of the vertical swing of the electron beam will remainat the center line of -the cathode ray tubeV screen 25.

' Assume now that the electron beam is sweeping past a portion 38 of theline mask 32 which is above the center line 45 of the cathode ray tubescreen 25. Assume further that this situation has just occured, and thatthe midpoint of the vertical swing of the electron beamk has not yetbeen corrected to the new conditions. It canbe visualizedrthat, under,these circumstances, as the electron beam is deected vertically it willsweep past the mask 32 near the top of its swing and above the zerocrossings. Morespecilically, assume that under these conditions theelectron beam sweeps by the line mask at points corre sponding to thepoints l and 61 of the vertical deecting signal represented by thewaveform of Figure 3. As a result pulses will be produced by thephotoelectric cell 30 such'as represented bythe pulse waveforms 62 and63 of Figure 5. There will be no pulses generated in the photoelectriccell 30 during the negative half cycle 59 of the waveform of Figure 3.However, Vtwo more pulses 64 and 65 will be generated by thephotoelectric cell 30ofFigure 1 during the positive half cycle 70 of thewaveform'of Figure 3. portions 54 and 70 of the waveform of Figure 3, atwhich time the tube 148 ofFigure 2 is conducting, the pulses 62,63, 64and 65 of Figure 5 will cause `the potential of the cathode of tube 148to decreasethusr increasing the current flow therethrough.Consequentlyrthe potential acquired by the capacitor 42 will becorrespondingly increased. However, during the vnegative half portion ofthe waveform of Figure 3, when the tube 149 is conductive, the currenttherethrough will be determined solely by the negative half cycle ofwaveforml 59. No pulses will be supplied from the photoelectric cell 30during this interval of time. Therefore the potential accumulated by thecapacitor 42 during a positive half cyclerof the waveform of Figure 3will be-greater than the potential acquired by the capacitor 43 during anegative half cycle ofthe waveform'of Figure 3 andV there will be aD.C.` outputvoltage across the conductors 44 and 53 whoseVmagnitude'varies in accordance with the distance between the portion ofthe mask being scanned and the center line 45. This output voltage,which in Figure l appears on conductor 18 is supplied to the addercircuit 29 where'it is combined with the signal from source 28. Thecombined signal output from the adder circuit 29 is supplied to thedeflection plates 24 to cause the mid-point of the vertical sweep of theelectron beam to move upward towards the portion of the line mask 32being swept by the-electron beam. It

can bevseen that, if the mid-point of the electron beam is Y movedupward to a point where it coincides exactly with the portion of theline mask being scanned, there will be no voltage output from the phasecomparator 31, and consequently no available voltage to maintain themid-point of the vertical sweep of the electron beam near the portion ofthe mask being scanned. The mid-point ofthe vertical sweep of the beammust remain a certain physical distance under the new position of theline mask 32 in order to generateV an output from the phase comparator31.

The magnitude of this distance is determined largely 'byV the gain ofthe feedback loop as discussed hereinbefore.

Assume now that the electron beam 'is caused to be swept across a thirdportion 39 o f Vthe linemask which is below the center line 45 ofthecathode ray tube screen 25. Assume further that the electron'beamhasnotyet had time to adjust to its new, corrected position. Under theseconditions'the electron beam will sweep by the line It can be seen then,that during the positive assaults 9 mask near the bottom of its verticalsweep. More specically, as shown in Figure 3, the electron beam willsweep by the new position 39 of the mask 32 yat times corresponding tothe points 68 and 69 of the negative half cycle 59 of the waveform ofFigure 3. Pulses such as pulses 66 and 67 of Figure 6 will thereby beproduced by the photoelectric element 36. However, no pulses will -beproduced in the output of the photoelectric element 30 during thepositive half cycles of the waveform of Figure 3 such as `positive halfcycles 54 and 70. Consequently, since the negative half cycle 50 of thewaveform of Figure 3 causes the anode 58 of the diode 149 to becomepositive, the current flow through the diode 149 during the time it isconductive will be greater than the current ow through the diode 148during the half cycles that it is conductive, and the potentialaccumulated on the capacitor 43 will be -greater than that accumulatedon the capacitor 42. A negative D C. potentail will consequently beproduced across conductors 44 and 53 whose magnitude varies inaccordance with distance between the portion of the mask being scannedand the center line 45. In Figure l the corresponding negative potentialappears on conductor 32, which corresponds to conductor 44 o-f Figure 2,and is supplied to the amplier 33, the output of which is in turnsupplied to the adder circuit 29. The adder circuit combines said signalwith the signal from the source 28 to produce an output signal having aD.C. component which will deilect the mid-point of the vertical sweep ofthe electron beam downward towards the portion 39 of the line mask 32.However, since a constant D.C. component from the ouptut of the addercircuit 29 is necessary to maintain the mid-point of the vertical sweepof the electron beam in this new position, the said midpoint cannotcoincide exactly with the point 39 of the line mask. Instead it will bepositioned above the point 39 of the line mask a distance sufficient toproduce an output from the phase comparator 31, which, when amplified,will maintain the mid-point of the vertical sweep of the electron beamin its new position as described hereinbefore.

Referring now to Figure 7, there is shown another form of the invention.Certain elements of the structure of Figure 7 correspond to similarelements of the structure of Figure l. These corresponding elements have`the same reference characters except that in Figure 7 the referencecharacters are primed. More specifically in Figure 7, input signalsource 28', input source 21', photoelectric cell 30', load 19', cathoderay tube 23 and the components thereof which include the cathode 26',focussing and accelerating anodes 27 and 34', batteries 35' and 36',Vertical deflecting plates 24', horizontal deflecting plates 22' and thescreen 25' correspond to elements of Figure l having the same unprimedreference characters. The principal difference between the circuit ofFigure 1 and the circuit of Figure 7 is that in Figure 7 there is nofeedback circuit from the phase comparator circuit 91 to the verticaldeflection plates 24' of the cathode ray tube 23' whereas such afeedback circuit comprising amplifier 33 and adder circuit 29 does existin the circuit of Figure 1. This difference in structure results in thefollowing consequences. In Figure 7 the mid-point of the vertical sweepof the electron beam stays at a certain horizontal line of the cathoderay tube,` which ordinarily would be chosen to correspond to the centerline 110 of the screen of the cathode ray tube 23. In Figure l, themid-point of the vertical sweep of the electron beam varies inaccordance with the position of the portion of the mask being scanned ata given time. The phase comparator circuit 91 of Figure 7 is similar to-that used in Figure 1. It is to be noted, however, that in Figures 1, 7and l2 suitable phase comparators other than the one shown in Figurek 2could be employed. Circuit limiter 115 provides for uniformity ofsignals supplied to phase comparator 91, thus improving the reliabilityof the device since the output of circuit 91 will thereby depend only onthe magnitude of the signal supplied from the vertical sweep signalsource 28. If desired current limiters may be used in correspondingportions of the circuits of Figures 1 and 12.

The operation of the circuit of Figure 7 will now be described. Assumethat the portion 107 of the line mask 111 being swept by the electronbeam is at the center line of the screen 25' of the cathode ray'tube.Assume further that, in the absence of a D.C. signal from the phasecomparator circuit 91, the mid-point of the vertical sweep of theelectron beam is at the center line of the cathode ray tube screen 25'.Under these conditions the electron beam will sweep by the mask at eachzero crossing of the vertical sweep signal from source 28'. Thisvertical sweep signal is represented by the waveform of Figure 8.Consequently pulses will be generated in the output of the photoelectriccell 30' corresponding in time to these zero crossings. Such pulses arerepresented by the pulse waveforms 154, 155, 156 and 157 of the waveformof Figure 9, and are supplied to the phase comparator circuit 91 in thesame manner as pulses from the photosensitive device. 30 of Figure l aresupplied to the phase comparator circuit 31. Since the pulses of Figure9 have a frequency equal to twice that of the waveform of Figure 8, andoccur at corresponding parts of the half cycles of Figure 8, the D.C.output of the phase comparator circuit will be zero as discussed withrespect to Figure 2.

Assume now that the electron beam of the tube 23 in Figure 7 is sweepingpast a portion such as portion 108 of the line mask 111 which lies abovethe horizontal center-line 110 of the screen of the tube. The times ofintersection of the electron beam and the mask are represented by points150,151, 152 and 153 of Figure 8. A voltage waveform vconsisting ofpulses such as pulses 158, 159, 160 and 161 in Figure 1-0 will begenerated at the output of the photoelectric cell 30'. It will beobserved that these output pulses occur only during the positive halfcycles of the waveform of Figure 8. Consequently, as discussed inconnection with Figure 2 there will be produced a positive outputvoltage from the phase comparator circuit 91.

Assume now that the electron beam is sweeping by a portion 109 of themask 111 which is positioned below the horizontal center line 116 of thetube 23. Under these conditions there will be produced, in the outputcircuit of the photoelectric cell 30', a series of pulses such as pulsewaveforms 162 and 163 shown in Figure 1l. These pulses are supplied tothe phase comparator circuit 91 of Figure 7 which is responsive theretoto produce a negative D.C. output voltage as described with respect toFigure 2.

Referring now to Figure 12 there is shown an embodiment of the inventionadapted to follow motion in two dimensions. The vertical sweep signalsource 117 and the horizontal sweep signal source 118 supply sweepVoltage signals to the vertical deection plates 119 and the horizontaldeflection plates 120 of the cathode ray tube 121 which also comprises acathode 122 and focussing and accelerating anodes representedrespectively by the reference characters 123 and 141. Batteries 143 and142 supply voltages for the focussing and accelerating anodes 123 and141. The vertical deection signal source 117 may be constructed toproduce a sine wave orrany other convenient waveform. Similarly thehorizontal deflection signal source 118 may be constructed to produce aconvenient waveform. The phase comparator circuits 124 and 125 aresimilar in structure and operation to that of the phase comparatorcircuit 31 of Figure l. More spef ciically the phase comparator circuit124 of Figure 12 functions to produce a signal whose amplitude isproportional to the difference in distance between the midpoint of thevertical sweep of the electron beam and the position of the spot mask126. The phase polarity comparator circuit 125 functions to produce anoutput signal whose amplitude varies in accordance with the differencein distance between the mid-point of the horizontal sweep of theelectron beam and the position of the Yspot mask 126. The mask 126,canbein the form'of a spot on a piece of transparent material such asglass, or it can be the point of a stylus moyingpabout on a transparentmaterial, such as a sheet of glass, which is positioned substantiallyparallel to the plane of the screen 1,28 of the tube 121.

Assume that the ratio of the frequency of the vertical deflection signalto the frequency of the horizontal deflection signal is large. ThisVwill result in a Vraster in which, each time the beam isdefiectedvertically overthe screen, it will move only a fraction of thedistance hori- Yzontally across the screen. The lightvappearing on Ythescreen of the cathode tube 121 will, 'of course, follow the pattern ofthe raster. Eachtime the mask 126 cuts 01T the light on thescreen'128.from the photoelectrc element 127. an output signal isgenerated by said photoelectric element 127. YThis signal is supplied tothe phase comparator circuits 124 and 125. Y n v i The outputV signalsfrom the phase comparator circuits '124 and 125 can be supplied tosuitable loads such as representedby reference characters 129 and 130respectively.

The operation of the circuit of Figure V1.2 willrnov'vi'b'e described.Figure 13 illustrates thepath of a typical raster which appears on-thescreen of the' tube 121 of Figure l2. The ordinate and abscissa ofFigure 13 represent dimensions. VMore specifically, point Y representsthevertical dimension of the screen 128 .of tube 121 .and point Xrepresents the horizontal dimension of the screen of tube 121. Thenumber of vertical sweeps in a framev will vordinarily be greater thanis shown in Figure 13. The particularwaveform shown on Figure 13 is usedonly for purposes of illustration. Point YM represents the mid-point ofthevertical sweepy and point XM representss the mid-pointV ofthehorizontal sweep. If a mask 131 is located at the Ymid-point of thescreen it will be at the mid-point of thevertical sweep and also at themid-point of the horizontalfsweep. Consequently the electron beam willsweep .pastV the mask at times corresponding to the zero lcrossing ofthe vertical sweep and also corresponding to the zero crossing of thehorizontal sweep. Thus, as discussed in connection with Figure 2 therewill be no output signal from the phase comparator circuits 124 and 125.Assume now that the point mask is moved to a position indicated byreference character 132. Under these circumstances the electron beamwill sweeprfby the mask 132'-above the zero crossing of both thevertical sweep and theV horizontal sweep. Signals will thereby begenerated. in both pulse comparator circuits 124 and 125. If the maskshould be located on the mid-point 'of `the `horizontal sweep, asrepresented by reference characterr133, there will be no output from thephase comparatorcircuitlZS. There will, however, be an output signalfrom thephase comparator circuit 124 inasmuch as ,the mask 133 is abovethe zero .crossing of theliverticalsignal. AIf the mask is positioned asindicatedby reference character 134, there will be no output signal fromthe phase polarity comparator circuit 124 lsince the mask corresponds tothe zero crossing of the vertical deflection sweepvsignal. However therewill be an output Vfrom the phase comparator circuit 125 sincethe masklies below the zero crossing of the horizontal deflection sweep signal.`Thus it can be seen that, as the mask 126 of Figure 12 is moved aroundthe surface of the screen ofthetube 121, the output signals from thephasecomparator Ycircuits v '124 and -125 will varyinaccordance with theinstantaneous position of the mask. Y

It is to beunderstood that the forms of the invention herein shown anddescribed are but preferred vembodiments of the same and that variouschanges may be made in the circuit constants used and inycircuitYarr-angement without departing from the scope of the invention.

t I claim: s l.. Yln a system for generating a signal whose amplitudevaries in accordance with ,the shape of a mask, means for generating anelectron beam, a light-emitting screen arranged to be impinged by saidbeam, first dellecting means responsive to a deflecting signal appliedthereto to deflect said beam in a first coordinate, second deflectingmeans responsive to a deectingsignal applied thereto to deflect saidbeam in a second coordinate, means for applying a first deectingrsignalto said first deflecting means to produce a cyclical deflection of saidbeam in said first coordinate about a nominal mid-point, means forapplying a second deflecting signal to said second dellecting means tocause said beam to be deflected in a Vsecond coordinate, a lightresponsive means exposed to light emitted by said screen and constructedto be responsive to variations in the intensity of said light to producean output signal, a mask positioned between said screen andsaid lightresponsive means, a'phase comparator circuit constructed torcombine thesignal from said light responsive means and the said first deflectingsignal to produce an output signal Whose magnitude varies in accordancewith the difference in distance in said first coordinate betweenv saidnominal` mid-point and that portion of theY mask intercepting said beam,and feedback means'responsive to the said output signal to produce asignal which is supplied therethrough to said first deiiecting means tomove. said mid-point in such a direction as to decreasesaid differencein distance. Y

2. An electrical system in accordance with claim 1 in which'saidfeedback means is constructed toV be responsive to said output signal toproduce a signal which will decrease said, difference in distance untilthe signal supplied to said first deflection means through said feedbackmeans is of the proper 4value to maintain the new position of saidmid-point.V v

3. lnQa system for generating a signal which/bears a known relationshipto the shape of a mask, means for 'generatinggan electron beam, alight-emitting screen arranged to be impinged by said beam, detiectingmeans responsive to a deflecting signal applied thereto to deflect saidbeam in a first coordinate, means for applying a deflecting signal tosaid detiecting means to produceV a deflection of said beam in saidfirst coordinate about a nominalV mid-point, a light responsive deviceexposed to light emitted by said screenv and constructed to produce anoutput signal whose magnitude varies in accordance with variations 'inthe intensity of said light, a mask positioned between said lightresponsive device and said screen, phase comparator means for combiningsaid deiiecting signal and the signal from said light responsive deviceto produce another output signal whose amplitude varies inl accordancewith theV position of the portion of the mask intercepting said beam,and feedback'means responsive tosaid other output signal to produce asignal which is supplied therethrough to said deflecting means to causesaid mid-point to Vassume a new position in close proximity in saidfirst coordinate to that portion of said mask intercepting said beam. 4.In a system for generating a signal Whose amplitude variesA inaccordance with the position of a spot mask, meansfor generating anelectron beam, a light-emitting screen arranged to be impinged by saidbeam, first deliection means responsive to a signal applied thereto todeflect said beam in a rst coordinate, second deflection meansresponsive to a signal applied thereto to deflect said beam in a secondcoordinatemeans for applying a first deliecting signal to said firstdeflection means to produce-.a cyclical deection of said beam in saidfirst coordinate, means for applying a second deectingsignal to saidsecond deflection means to produce a Acyclical deflection of said beamin said second coordinate, light responsive means exposed to lightemitted by said'screen and'constructed to produce an output signal whosemag- 13 tensity of said light, a spot mask positioned between said lightresponsive means and said screen, first phase comparator circuit meansconstructed to combine said first deectingsignal and the signal fromsaid light responsive means to produce a second output signal whosemagnitude varies in accordance with the position of said spot mask insaid first coordinate, and second phase comparator circuit meansconstructed to combine the said second deflecting signal with the signalfrom said light responsive means to produce a third output signal Whosemagnitude varies in accordance with the position of said spot mask insaid second coordinate.

5. In -an electrical system for generating a signal whose amplitudevaries in accordance with the shape of a mask, a cathode ray tubecomprising a luminous screen, means for generating an electron beam,first deflecting means responsive to a deecting signal applied theretoto deflect said electron beam in a first coordinate, and seconddeflecting means responsive to a defiecting signal applied thereto todeflect said electron beam in a second coordinate, means for applying afirst deecting signal to said first deecting means to cause saidelectron beam yto cyclically sweep across at least a portion of saidscreen in said first coordinate about a nominal mid-point, means forapplying a second deliecting signal to said second deecting means tocause said electron beam to be deflected across at least a portion ofsaid screen in a second coordinate, a light responsive device exposed tothe light from said luminous screen and constructed to produce an outputsignal whose magnitude varies in accordance with variations in theintensity of light from said luminous screen, a mask positioned betweensaid screen and said light responsive device, a phase comparator circuitconstructed to combine the signal from said light responsive device andthe said first deflecting signal to produce a second output signal whosemagnitude varies in accordance with the difference in distance in saidfirst coordinate between said nominal mid-point and that portion of themask intercepting the light between said screen and said lightresponsive device, and feedback means responsive to the said secondoutput signal to produce a signal which is supplied therethrough to saidfirst defiecting means to move said mid-point in such a direction as todecrease said difference in distance.

6. In an electrical system for generating a signal whose amplitudevaries in accordance with the position of a spot mask, a cathode raytube comprising a luminous screen, means for generating an electronbeam, first deflection means responsive to a signal applied thereto todeect said electron beam in a first coordinate, second deflection meansresponsive to a signal applied thereto to deiiect said electron beam ina second coordinate, means for applying a first detiecting signal tosaid first deflection means to cause said electron beam to sweepcyclically across said luminous screen in said first coordinate, meansfor applying a second deflecting signal to said second deflection meansto cause said electron. beam to sweep cyclically across said screen insaid second coordinate, light responsive means exposed to the light fromsaid luminous screen and constructed to produce an output signal whosemagnitude varies in accordance with variations in the intensity of thelight from said screen, a spot mask positioned between said luminousscreen and said light responsive means, first phase cornparator circuitmeans constructed to combine said first deflecting signal and the outputsignal from said light responsive means to produce a second outputsignal whose magnitude varies in accordance with the position of saidspot mask in said first coordinate, and second phase comparator circuitmeans constructed to combine said second deiiecting signal with theoutput signal from light responsive means to produce a third outputsignal whose magnitude varies in accordance with the position of saidmask in said second coordinate.

7. In an electrical signal generating system, means for mme producing anelectron beam, a light-emitting screen arf ranged to be impinged by saidbeam, first deflecting means responsive to a deflecting signal appliedthereto to deflect said beam in a first coordinate, second deliectingmeans responsive to a deflecting signal applied thereto to deiiect saidbeam-in a second coordinate, means for applying to said first deiiectingmeans a first deflecting signal which causes said beam to move in saidfirst coordinate cyclically and symmetrically with respect to a nominalaxis at la relatively high rate, means for applying to said seconddeecting means a second deliecting signal which causes said beam to movein said second coordinate at a relatively lower rate as it movescyclically with respect to said axis, a light responsive device exposedto the light emitted by said screen and adapted to produce a signalwhose magnitude varies in accordance with variations in the intensity oflight from said screen, a masking element positioned between said screenand said light responsive device, and phase comparator means forcornbining said first defiecting signal and the signal from said lightresponsive device to produce an output signal whose amplitude variesaccording to light interception by said masking element in relation tosaid nominal axis.

8. In an electrical system for generating a signal whose amplitudevaries in accordance with a mask, a cathode ray tube comprising aluminous screen, means for generating an electron beam, first deflectingmeans responsive to a defiecting signal applied thereto to deflect saidelectron beam in a first coordinate, and second deflecting meansresponsive to a defiecting signal applied thereto to deflect saidelectron beam in a second coordinate, means for applying to said firstdefiecting means a first defiecting signal which causes said electronbeam to move in said first coordinate cyclically and symmetrically withrespect to a nominal axis at a relatively high rate, means for applyingto said second defiecting means a second deflecting signal which causessaid beam to move in said second coordinate at a relatively lower rateas it moves cyclically with respect to said axis, a light responsivedevice exposed to the light from said luminous screen and constructed toproduce a signal whose magnitude varies in accordance with variations inthe intensity of light from said luminous screen, a mask positionedbetween said screen and said light responsive device, and a phasecomparator circuit constructed to combine the signal from said lightresponsive device and said first deecting signal to produce an outputsignal whose magnitude varies in accordance with the distance in saidfirst coordinate between said nominal axis and that portion of the maskintercepting the light between said screen and said light responsivedevice.

9. A system according to claim 8, wherein said first deecting meansdefiects said electron beam vertically and said second deiiecting meansdeilects said electron beam horizontally.

10. In an electrical system for generating a signal whose amplitudevaries in accordance with a mask, a cathode ray tube comprising aluminous screen, means for generating an electron beam, first deflectingmeans responsive t-o a deiiecting signal applied thereto to deflect saidelectron beam in a first coordinate, and second deiiecting meansresponsive to a deflecting signal applied thereto to deflect saidelectron beam in a second coordinate, means for applying to said firstdeflecting means a first defiecting signal which causes said electronbeam to move in said first coordinate cyclically and symmetrically withrespect to a nominal axis at a relatively high rate, means for applyingto said second defiecting means a second deflecting signal which causessaid beam to move in said second coordinate at a relatively lower rateas it moves cyclically with respect to said axis, a light responsivedevice exposed to the light from said luminous screen and constructed toproduce a signal whose magnitude varies in accordance withv variationsin the intensity of light from said luminous V15 screen, a maskpositioned ybetween said screen and rsaid light responsive device,a-phase comparator circuit constructed to combine the signal from saidlight responsive device and said rst deecting signal to produce anoutput signal whose magnitude varies in accordance with the distance insaidrst coordinate between said nominal `axis and that portion of themask interceptng the light betweensaid screen and said light responsivedevice, and a phase comparator circuit constructed to combine the signalfrom said light responsive device and said second deflecting signal toproduce a second output signal whose magnitude varies in accordance withthe distance in said second coordinate between a nominal axis and thatportion of the mask lintercepting the light between said screen and'saidlightresponsive device.

References Cited in the tile of this patentl UNITED :STATES PATENTS2,489,305 McClennan Nov. 29, 1949 2,499,178 Berry et a1 Feb. 28, 19502,528,020 Sunstein Oct. 31, 1950 2,656,101 Haviland Oct. 20, 19532,701,850

Blayney Feb. 8, 1955

